Positive ignition engines cause combustion of a hydrocarbon and air mixture using spark ignition. Contrastingly, compression ignition engines cause combustion of a hydrocarbon by injecting the hydrocarbon into compressed air. Positive ignition engines can be fuelled by gasoline fuel, gasoline fuel blended with oxygenates including methanol and/or ethanol, liquid petroleum gas or compressed natural gas. Positive ignition engines can be stoichiometrically operated engines or lean-burn operated engines.
TWCs are intended to catalyse three simultaneous reactions: (i) oxidation of carbon monoxide to carbon dioxide, (ii) oxidation of unburned hydrocarbons to carbon dioxide and water; and (iii) reduction of nitrogen oxides to nitrogen and oxygen. These three reactions occur most efficiently when the TWC receives exhaust gas from an engine running at or about the stoichiometric point. As is well known in the art, the quantity of carbon monoxide (CO), unburned hydrocarbons (HC) and nitrogen oxides (NOx) emitted when gasoline fuel is combusted in a positive ignition (e.g. spark-ignited) internal combustion engine is influenced predominantly by the air-to-fuel ratio in the combustion cylinder. An exhaust gas having a stoichiometrically balanced composition is one in which the concentrations of oxidising gases (NOx and O2) and reducing gases (HC and CO) are substantially matched. The air-to-fuel ratio that produces this stoichiometrically balanced exhaust gas composition is typically given as 14.7:1.
Theoretically, it should be possible to achieve complete conversion of O2, NOx, CO and HC in a stoichiometrically balanced exhaust gas composition to CO2, H2O and N2 (and residual O2) and this is the duty of the TWC. For this purpose a three-way catalyst (TWC) typically contains one or more platinum group metals, particularly those selected from the group consisting of platinum, palladium and rhodium. Ideally, therefore, the engine should be operated in such a way that the air-to-fuel ratio of the combustion mixture produces the stoichiometrically balanced exhaust gas composition.
A way of defining the compositional balance between oxidising gases and reducing gases of the exhaust gas is the lambda (λ) value of the exhaust gas, which can be defined according to equation (1) as:Actual engine air-to-fuel ratio/Stoichiometric engine air-to-fuel ratio,  (1)wherein a lambda value of 1 represents a stoichiometrically balanced (or stoichiometric) exhaust gas composition, wherein a lambda value of >1 represents an excess of O2 and NOx and the composition is described as “lean” and wherein a lambda value of <1 represents an excess of HC and CO and the composition is described as “rich”. It is also common in the art to refer to the air-to-fuel ratio at which the engine operates as “stoichiometric”, “lean” or “rich”, depending on the exhaust gas composition which the air-to-fuel ratio generates: hence stoichiometrically-operated gasoline engine or lean-burn gasoline engine.
It should be appreciated that the reduction of NOx to N2 using a TWC is less efficient when the exhaust gas composition is lean of stoichiometric. Equally, the TWC is less able to oxidise CO and HC when the exhaust gas composition is rich. The challenge, therefore, is to maintain the composition of the exhaust gas flowing into the TWC at as close to the stoichiometric composition as possible.
Of course, when the engine is in steady state it is relatively easy to ensure that the air-to-fuel ratio is stoichiometric. However, when the engine is used to propel a vehicle, the quantity of fuel required changes transiently depending upon the load demand placed on the engine by the driver. This makes controlling the air-to-fuel ratio so that a stoichiometric exhaust gas is generated for three-way conversion particularly difficult. In practice, the air-to-fuel ratio is controlled by an engine control unit, which receives information about the exhaust gas composition from an exhaust gas oxygen (EGO) (or lambda) sensor: a so-called closed loop feedback system. A feature of such a system is that the air-to-fuel ratio oscillates (or perturbates) between slightly rich of the stoichiometric (or control set) point and slightly lean, because there is a time lag associated with adjusting air-to-fuel ratio. This perturbation is characterised by the amplitude of the air-to-fuel ratio and the response frequency (Hz).
The active components in a typical TWC comprise one or both of platinum and palladium in combination with rhodium, or even palladium only (no rhodium), supported on a high surface area oxide, and an oxygen storage component.
When the exhaust gas composition is slightly rich of the set point, there is a need for a small amount of oxygen to consume the unreacted CO and HC. i.e. to make the reaction more stoichiometric. Conversely, when the exhaust gas goes slightly lean, the excess oxygen needs to be consumed. This was achieved by the development of the oxygen storage component that liberates or absorbs oxygen during the perturbations. The most commonly used oxygen storage component (OSC) in modern TWCs is cerium oxide (CeO2) or a mixed oxide containing cerium, e.g. a Ce/Zr mixed oxide.
Ambient PM is divided by most authors into the following categories based on their aerodynamic diameter (the aerodynamic diameter is defined as the diameter of a 1 g/cm3 density sphere of the same settling velocity in air as the measured particle):                (i) PM-10—particles of an aerodynamic diameter of less than 10 μm;        (ii) Fine particles of diameters below 2.5 μm (PM-2.5);        (iii) Ultrafine particles of diameters below 0.1 μm (or 100 nm); and        (iv) Nanoparticles, characterised by diameters of less than 50 nm.        
Since the mid-1990's, particle size distributions of particulates exhausted from internal combustion engines have received increasing attention due to possible adverse health effects of fine and ultrafine particles. Concentrations of PM-10 particulates in ambient air are regulated by law in the USA. A new, additional ambient air quality standard for PM-2.5 was introduced in the USA in 1997 as a result of health studies that indicated a strong correlation between human mortality and the concentration of fine particles below 2.5 μm.
Interest has now shifted towards nanoparticles generated by diesel and gasoline engines because they are understood to penetrate more deeply into human lungs than particulates of greater size and consequently they are believed to be more harmful than larger particles, extrapolated from the findings of studies into particulates in the 2.5-10.0 μm range.
Size distributions of diesel particulates have a well-established bimodal character that correspond to the particle nucleation and agglomeration mechanisms, with the corresponding particle types referred to as the nuclei mode and the accumulation mode respectively (see FIG. 1). As can be seen from FIG. 1, in the nuclei mode, diesel PM is composed of numerous small particles holding very little mass. Nearly all diesel particulates have sizes of significantly less than 1 μm, i.e. they comprise a mixture of fine, i.e. falling under the 1997 US law, ultrafine and nanoparticles.
Nuclei mode particles are believed to be composed mostly of volatile condensates (hydrocarbons, sulfuric acid, nitric acid etc.) and contain little solid material, such as ash and carbon. Accumulation mode particles are understood to comprise solids (carbon, metallic ash etc.) intermixed with condensates and adsorbed material (heavy hydrocarbons, sulfur species, nitrogen oxide derivatives etc.) Coarse mode particles are not believed to be generated in the diesel combustion process and may be formed through mechanisms such as deposition and subsequent re-entrainment of particulate material from the walls of an engine cylinder, exhaust system, or the particulate sampling system. The relationship between these modes is shown in FIG. 1.
The composition of nucleating particles may change with engine operating conditions, environmental condition (particularly temperature and humidity), dilution and sampling system conditions. Laboratory work and theory have shown that most of the nuclei mode formation and growth occur in the low dilution ratio range. In this range, gas to particle conversion of volatile particle precursors, like heavy hydrocarbons and sulfuric acid, leads to simultaneous nucleation and growth of the nuclei mode and adsorption onto existing particles in the accumulation mode. Laboratory tests (see e.g. SAE 980525 and SAE 2001-01-0201) have shown that nuclei mode formation increases strongly with decreasing air dilution temperature but there is conflicting evidence on whether humidity has an influence.
Generally, low temperature, low dilution ratios, high humidity and long residence times favour nanoparticles formation and growth. Studies have shown that nanoparticles consist mainly of volatile material like heavy hydrocarbons and sulfuric acid with evidence of solid fraction only at very high loads.
Contrastingly, engine-out size distributions of gasoline particulates in steady state operation show a unimodal distribution with a peak of about 60-80 nm (see e.g. FIG. 4 in SAE 1999-01-3530). By comparison with diesel size distribution, gasoline PM is predominantly ultrafine with negligible accumulation and coarse mode.
Particulate collection of diesel particulates in a diesel particulate filter is based on the principle of separating gas-borne particulates from the gas phase using a porous barrier. Diesel filters can be defined as deep-bed filters and/or surface-type filters. In deep-bed filters, the mean pore size of filter media is bigger than the mean diameter of collected particles. The particles are deposited on the media through a combination of depth filtration mechanisms, including diffusional deposition (Brownian motion), inertial deposition (impaction) and flow-line interception (Brownian motion or inertia).
In surface-type filters, the pore diameter of the filter media is less than the diameter of the PM, so PM is separated by sieving. Separation is done by a build-up of collected diesel PM itself, which build-up is commonly referred to as “filtration cake” and the process as “cake filtration”.
It is understood that diesel particulate filters, such as ceramic wallflow monoliths, may work through a combination of depth and surface filtration: a filtration cake develops at higher soot loads when the depth filtration capacity is saturated and a particulate layer starts covering the filtration surface. Depth filtration is characterized by somewhat lower filtration efficiency and lower pressure drop than the cake filtration.
Other techniques suggested in the art for separating gasoline PM from the gas phase include vortex recovery.
Emission legislation in Europe from 1 Sep. 2014 (Euro 6) requires control of the number of particles emitted from both diesel and gasoline (positive ignition) passenger cars. For gasoline EU light duty vehicles the allowable limits are: 1000 mg/km carbon monoxide; 60 mg/km nitrogen oxides (NOx); 100 mg/km total hydrocarbons (of which ≦68 mg/km are non-methane hydrocarbons); and 4.5 mg/km particulate matter ((PM) for direct injection engines only). The Euro 6 PM standard will be phased in over a number of years with the standard from the beginning of 2014 being set at 6.0×1012 per km (Euro 6) and the standard set from the beginning of 2017 being 6.0×1011 per km (Euro 6+). In a practical sense, the range of particulates that are legislated for are between 23 nm and 3 μm.
In the United States, on 22 Mar. 2012, the State of California Air Resources Board (CARB) adopted new Exhaust Standards from 2017 and subsequent model year “LEV III” passenger cars, light-duty trucks and medium-duty vehicles which include a 3 mg/mile emission limit, with a later introduction of 1 mg/mi possible, as long as various interim reviews deem it feasible.
The new Euro 6 (Euro 6 and Euro 6+) emission standard presents a number of challenging design problems for meeting gasoline emission standards. In particular, how to design a filter, or an exhaust system including a filter, for reducing the number of PM gasoline (positive ignition) emissions, yet at the same time meeting the emission standards for non-PM pollutants such as one or more of oxides of nitrogen (NOx), carbon monoxide (CO) and unburned hydrocarbons (HC), all at an acceptable back pressure, e.g. as measured by maximum on-cycle backpressure on the EU drive cycle.
It is envisaged that a minimum of particle reduction for a three-way catalysed particulate filter to meet the Euro 6 PM number standard relative to an equivalent flowthrough catalyst is ≧50%. Additionally, while some backpressure increase for a three-way catalysed wallflow filter relative to an equivalent flowthrough catalyst is inevitable, in our experience peak backpressure over the MVEG-B drive cycle (average over three tests from “fresh”) for a majority of passenger vehicles should be limited to <200 mbar, such as <180 mbar, <150 mbar and preferably <120 mbar e.g. <100 mbar.
There have been a number of recent efforts to combine TWCs with filters for meeting the Euro 6 emission standards.
US 2009/0193796 discloses a zoned oxidation catalyst disposed on a soot filter, wherein an inlet zone of 50% of the total axial length of the soot filter is coated with an inlet coat comprising platinum and palladium at a metal loading of 60 gft−3 supported on lanthanum-stabilised high surface area gamma alumina, a ceria-zirconia composite and zirconium oxide at a washcoat loading of 0.64 gin−3; and an outlet zone of 50% of the total axial length of the soot filter is coated with an outlet coat also comprising platinum and palladium at a metal loading of 15 gft−3 supported on lanthanum-stabilised high surface area gamma alumina, a ceria-zirconia composite and zirconium oxide at a washcoat loading of 0.61 gin−3. The total precious metal loading in the soot filter was 37.5 gft−3 and the Pt/Pd/Rh ratio was 25/12.5/0.
Catalysts for vehicular exhaust gas aftertreatment, such as three-way catalysts for simultaneously converting carbon monoxide, unburned hydrocarbons and oxides of nitrogen in exhaust gas emitted from a positive ignition internal combustion engine, can become deactivated through use. A primary cause of deactivation results from contamination (poisoning) by contaminants present in the feed gas.
There are two basic mechanisms where catalysts for treating exhaust gas from internal combustion engines can become poisoned: (i) selective poisoning, in which a contaminant reacts directly with an active site or catalyst support causing a reduction in activity or a catastrophic loss in activity; and (ii) non-selective poisoning, which causes a loss of performance by sterically hindering access to active sites or pores in a catalyst support by fouling (or masking) a surface of the support or active sites. An example of mechanism (ii) is the deposition of ash derived from the combustion of lubricant oil and fuel additives or coking by hydrocarbons. Build-up of ash derived e.g. from fuel additives can contribute to an increase in back pressure in the system and an increase in fuel consumption. From his experience, the inventor found that manganese, zinc, calcium and (at low temperature) phosphorus (as phosphoric acid droplets) and oil droplets as such (all derived from the fuel or lubricants) are non-selective poisons. Poisons which chemically react with catalyst components (mechanism (i)) include lead and sulphur oxides (by chemisorption) and (at higher temperature) phosphorus. A review of poisons and poisoning mechanisms can be found, for example, in A. J. J. Wilkins et al., Platinum Metals Review, 1990, 34(1) 16-24.
It is understood that ash which causes non-selective poisoning is introduced into diesel filters in combination with soot particles, which naturally collect towards the rear of the filter. The inventor noticed that where a three-way catalyst is coated on a flowthrough honeycomb substrate, poisons residues are concentrated close to the inlet, because they are primarily transported in droplets (e.g. oil, phosphoric acid) which decompose quickly when in first contact with the washcoat surface, depositing an inorganic residue. Very surprisingly, the inventor found that where the three-way catalyst is coated on a wall-flow filter substrate, a majority of poisoning occurs in a similar way to that seen on a TWC coated on a flowthrough honeycomb substrate, rather than poison transport in dry soot/ash, as is seen in diesel filters.
From this observation, the inventor had the idea to coat the inlet end of filters such as wallflow filters with a washcoat that preferentially traps catalyst poisons in order to protect catalyst washcoat further downstream in the filter, e.g. further downstream in the inlet channels of a wallflow filter and in the outlet channels.
The current Euro stage 5 emission standard requires a vehicle in-service conformity of 100,000 km or 5 years with durability testing of pollution control devices for type approval of 160,000 km or 5 years (whichever occurs first). In lieu of a durability test, manufacturers may use the following deterioration factors: Positive ignition engines: 1.5 for CO; 1.3 for HC; 1.6 for NOx; 1.0 for PM and PN; and for compression ignition, Euro 5: 1.5 for CO; 1.1 for NOx and HC+NOx; 1.0 for PM and PN. The deterioration factors for the future Euro 6 stage has yet to be determined.
Therefore there is a need in this technical field to reduce or prevent deterioration in performance of catalysed filter exhaust gas aftertreatment devices for positive ignition engines and increases in backpressure in an exhaust system for positive ignition engines that includes a filter via known modes of catalyst poisoning and fouling. The present invention is aimed at such need.
The invention proposes a solution to this need which is the application of a high washcoat loading/high specific surface area coating to an inlet zone of a filter, to preferentially trap incoming oil and other residues transported in droplets thereby to minimise poisons/ash build up on the remainder of the unit, thus maintaining catalyst activity to meet in-service conformity but minimising backpressure increase over the filter lifetime. An advantage of minimising washcoat permeability at the filter inlet (to trap poisons) is that there is less impact on backpressure than if reduced washcoat permeability were disposed toward the downstream end of the filter.